To compare the effectiveness of two dosage regimens of aerosolized pentamidine, 300 mg administered once monthly or 600 mg administered once monthly, as prophylaxis in patients at high risk for the development of Pneumocystis Carinii pneumonia. To study the plasma and bronchopulmonary pharmacokinetics of these dosage regimens using a highly specific and sensitive HPLC assay. To assess and compare long-term (1 yr) pulmonary toxicity of 600 mg once-monthly aerosolized pentamidine with the currently recommended dose of 300 mg once-monthly.